1. Field of the Invention
The present invention relates to an electronic apparatus having a speaker unit incorporated therein, such as a speaker apparatus, a monitor receiver or a radio receiver including at least one of a variety of display devices such as a plasma display device.
2. Description of the Related Art
A speaker apparatus produces a sound when a cone of a speaker unit thereof vibrates back and forth and generates a compressional wave in a space. When the cone moves forward, air in front of the cone is compressed, and air at the back of the same is quite contrarily rarefied. The speaker unit includes a cone and a magnetic circuit section, and the speaker apparatus means an overall apparatus including a speaker cabinet having the speaker unit incorporated therein.
When the speaker unit is sounded without attaching to a baffle, compressional states of air in front of and at the back of the unit are mixed and cancelled with each other, resulting in producing no sound. Although the-speaker unit may be attached to a large baffle so as to prevent sounds in front of and at the back of the unit, having mutually reverse phases, from being mixed with each other, a baffle effective to a low-frequency sound is enormously large in size and is accordingly unpractical.
In view of this problem, as shown in FIG. 13 by way of example, a bass-reflex speaker cabinet is hitherto used, having a structure in which a sound emitted from the rear of a cone of a speaker unit 901 passes through a bass-reflex duct (port, aperture) 902 disposed in a cabinet 900 so as to have a phase reversed by 180 degrees, in agreement with that of a sound emitted in front of the cone, and is then emitted from the speaker unit. While requiring a severe design, the bass-reflex speaker cabinet has advantages of, for example, a smaller size and much more satisfactorily playing back bass sound than an enclosed one.
FIG. 13A and FIG. 13B are respectively a sectional view and an elevational view of an example bass-reflex speaker apparatus. The bass-reflex speaker apparatus shown in FIG. 13 has an equivalent circuit shown in FIG. 14. In this case, the lowest resonant frequency f0 of the speaker unit 901 and the resonant frequency f0b of the speaker cabinet 900 are respectively given by expressions (1) and (2) shown in FIG. 15.
With respect to references characters shown in FIG. 14, Fv defines a vibromotive force exerted on a speaker vibrating system; Ev defines a voltage applied on a voice coil, B defines a flux density in a gap; l and Rv respectively define a length and a direct current resistance of the voice coil; vd defines a speed of a cone, vp defines a speed of air in a bass-reflex duct; m0, s0, and r0 respectively define an equivalent mass, an equivalent stiffness, and an equivalent resistance of the speaker vibrating system; sc defines an equivalent stiffness of a cabinet, vc defines a speed of air in the cabinet, and ml defines an equivalent mass of the duct.
For example, a bass-reflex speaker apparatus disclosed in Japanese Unexamined Patent Application Publication NO. 6-233367 has a structure in which, by making use of arrangement of a bass-reflex duct in a cabinet, a ventilation duct usable also as a bass-reflex duct is disposed in the vicinity of a yoke constituting the magnetic circuit of the apparatus so as to cool the magnetic circuit by air flowing in and out the ventilation duct and also to provide an bass-reflex effect.
While an audio amplifier is additionally needed to drive a speaker apparatus and is generally disposed independently from the speaker apparatus, another speaker apparatus having a structure in which an audio amplifier and so forth are built in its speaker cabinet in order to meet the requirements of reducing the size and saving a space of an audio system is also available in the market.
Also, of monitor receivers such as a television receiver including a display device such as a cathode ray tube (CRT), a liquid crystal display device, or a plasma display device, one having a structure in which a speaker unit is incorporated in its cabinet is also available in the market. With this structure, the speaker apparatus is not required to be disposed together with the monitor receiver and is accordingly useful.
In the meantime, with respect to a speaker apparatus or a monitor receiver having a speaker unit and additionally a variety of electrical circuits, built therein, since heat generated by the built-in electrical circuits remains in its cabinet having the speaker unit incorporated therein, such an electronic apparatus requires an elaborate design effort so as to effectively convey the heat remaining in the cabinet outside the apparatus.
To this end, the apparatus is designed so as to have an example structure in which a heat pipe, a thermally conductive material or the like is connected to a heating section of an amplifier circuit or the like so as to release heat in the cabinet having the speaker unit incorporated therein outside the apparatus. Unfortunately, this structure causes a complicated mechanism and also an increased cost. In addition, since as the greater the output power of an amplifier, the greater amount of generated heat, the maximum power of the amplifier is limited, thereby inhibiting a large sound from being played back.
In order to solve the above problem, disposing a ventilation duct serving as a ventilation path and also a bass-reflex duct in the speaker cabinet as disclosed in the foregoing patent document is conceivable. Unfortunately, only air flowing in and out the same ventilation duct does not effectively cool the inside of the cabinet. Especially, in the case of an electronic apparatus such as a speaker apparatus or a monitor receiver having a variety of electrical circuits incorporated therein, only air flowing in and out in its ventilation duct does not sufficiently cool the inside of the cabinet thereof.